1. Field of the Invention
This invention relates to a process for making surface mountable electrical devices.
2. Description of the Related Art
FIG. 1 illustrates a conventional surface mountable electrical device, such as a PTC thermistor, which includes a plurality of stacked PTC sheets 4 that have a positive thermal coefficient characteristic, a plurality of metal foil sheets 21, 23 alternately laminated with the PTC sheets 4, a first electrode layer 22 formed on an upper surface of the device, a second electrode layer 24 formed on a bottom surface of the device, a conductive first side layer 51 interconnecting the first electrode layer 22 and one of the metal foil sheets 23, and a conductive second side layer 52 interconnecting the second electrode layer 24 and another one of the metal foil sheets 21. Commercially available metal foil sheets, which are made from copper, normally have been surface treated at one side face so as to be attachable to a contacting surface of a PTC sheet. However, the other side face of each of the metal foil sheets is not surface treated and is relatively smooth. As a result, the non-surface-treated side face of each of the aforesaid metal foil sheets 21, 23 is in poor contact with the respective PTC sheet 4, and thus tends to peel from the respective PTC sheet 4.
In order to overcome the aforementioned drawback, U.S. Pat. No. 6,157,289 disclosed a surface mountable electrical device (see FIG. 2) that includes a metal foil sheet 11, which is sandwiched between two stacked PTC sheets 14 and which is surface treated by plating with nickel layers 12 on opposite side faces thereof so as to roughen the side faces and so as to permit secure attachment of the same to contacting surfaces of the PTC sheets 14. However, the extra processing step of forming the nickel layers 12 significantly increases the cost of manufacture. Moreover, the metal foil sheet 11 is required to be formed with grooves along cutting lines by etching or die press prior to being laminated with the PTC sheets 14 for facilitating formation of pieces of the surface mountable electrical device in a subsequent cutting step.
Therefore, the object of the present invention is to provide a process for making surface mountable electrical devices that is capable of overcoming the aforementioned drawbacks.
According to the present invention, a process for making surface mountable electrical devices comprises the steps of: preparing at least first and second PTC sheets which have a positive thermal coefficient characteristic and each of which has opposite first and second contact faces; respectively covering the first and second contact faces of each of the first and second PTC sheets with outer and inner metal foil sheets; selectively masking the inner metal foil sheet on each of the first and second PTC sheets to define a pattern of exposed areas on the inner metal foil sheet; removing portions of the inner metal foil sheet on each of the first and second PTC sheets at the exposed areas so as to define a pattern of non-inlaid portions of the second contact face of each of the first and second PTC sheets, the non-inlaid portions being exposed from the inner metal foil sheets; laminating the first and second PTC sheets and the outer and inner metal foil sheets to form a laminate such that the non-inlaid portions of the second contact face of the first PTC sheet are respectively bonded to the non-inlaid portions of the second contact face of the second PTC sheet and that the inner metal foil sheet on the first PTC sheet overlaps the inner metal foil sheet on the second PTC sheet; selectively masking the outer metal foil sheet on the first contact face of the first PTC sheet to define a pattern of exposed areas on the outer metal foil sheet on the first PTC sheet; removing portions of the outer metal foil sheet on the first PTC sheet at the exposed areas so as to define a pattern of slits in the outer metal foil sheet on the first PTC sheet; forming a plurality of bores in the laminate along intersecting cutting lines at locations where the cutting lines intersect, each of the bores extending from the outer metal foil sheet on the first PTC sheet to the outer metal foil sheet on the second PTC sheet in a transverse direction relative to the first and second contact faces; forming a conductive transverse layer in an interior of each of the bores such that the transverse layer extends from the outer metal foil sheet on the first PTC sheet to the outer metal foil sheet on the second PTC sheet; and cutting the laminate along the cutting lines to form a plurality of the surface mountable electrical devices such that the outer metal foil sheet on the first PTC sheet on each of the surface mountable electrical devices forms opposing first and second segments which are spaced apart by a respective one of the slits and that the transverse layer on each of the surface mountable electrical devices is electrically connected to the first segment of the outer metal foil sheet on the first PTC sheet, the inner metal foil sheets, and the outer metal foil sheet on the second PTC sheet.